Theelevationinextracellular[K+]causesdepolarizationofskeletalmusclecells.Initially,thedepolarizationbringsmusclecellsclosertothreshold,andsospontaneousactionpotentialsandcontractionsaremorelikelytooccur.Asdepolarizationofthecellsbecomesmoremarked,thecellsbecomerefractorybecauseincreasingnumbersofNa+ channelsbecomeinactivated.Consequently,thecellsbecomeunabletofireactionpotentialsandarenotabletocontractinresponsetoactionpotentialsintheirmotoraxons.
During the latter part of the action potential, and during the afterhyperpolarization period, the cell is able to fire a second action potential, but a stimulus stronger than normal is required. This period is called the relative refractory period. Early in the relative refractory period, before the
CHAPTER 5 Generation and Conduction of Action Potentials membrane potential has returned to the resting potential level, some Na+ channels are still voltage inactivated, but there are enough in the closed state (and therefore have the potential to open when the membrane is depolarized) to support the generation of an action potential if they are stimulated to open. However, a stimulus stronger than normal is necessary to recruit the critical number of Na+ channels needed to trigger an action potential (i.e., the reduction in the total number of available Na+ channels is countered by increasing the probability of opening). Throughout the relative refractory period, conductance to K+ is elevated, which opposes depolarization of the membrane. This increase in K+ conductance continues throughout the afterhyperpolarization and accounts for most of the duration of the relative refractory period.
Conduction of Action Potentials
Fundamental to nervous system function is the transmission of information along neuronal pathways. To accomplish this, neurons generate action potentials that propagate down the length of their axon without decrement in size in order to trigger neurotransmitter release from the presynaptic terminals. How action potentials propagate down an axon and how the characteristics of the axon affect this propagation are discussed in this section. How they trigger transmitter release is covered in
Action Potential as a Self-Reinforcing Signal
Passive conduction will not transport a signal from one end of an axon to the other unless the axon is very short (i.e., on the order of its length constant) because passively conducted signals decrease in size rapidly with distance from their origin. Neurons with such short axons exist; for example, in the retina of the eye, the distance from one neuron to the next is so small that electrotonic (passive) conduction is sufficient. However, in most cases, axons are many times longer than their length constant. In fact, they can be up to 1 m or more in length (e.g., those of motor neurons) and thus hundreds of times their length constant. Nonetheless, if researchers were to record from points along a typical axon, they would find that as the action potential arrives at successive points traveling along the axon, its shape and size remain constant. This is because the action potential regenerates itself as it is conducted along the fiber and thus is said to be actively propagated.